Multi-Level Pulse Width Modulation Power Amplifier Method and Apparatus
A baseband signal is processed by amplifying a first pulse width modulation radio frequency signal having a first non-negligible peak-to-peak amplitude and a second non-negligible peak-to-peak amplitude larger than the first non-negligible peak-to-peak amplitude. A second pulse width modulation radio frequency signal is also amplified, the second pulse width modulation radio frequency signal having a third non-negligible peak-to-peak amplitude approximately equal to the second non-negligible peak-to-peak amplitude of the first pulse width modulation signal when the baseband signal power is at or above the second threshold level. The amplified signals are constructively combined to form a pulse width modulation radio frequency signal comprising a plurality of non-negligible peak-to-peak amplitude levels each corresponding to a different baseband signal power range.
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The present invention generally relates to signal modulation, and particularly relates to imparting signal modulations by driving multiple Radio Frequency (RF) power amplifiers with single or multi-level Pulse Width Modulation (PWM) input signals, and combining their output signals efficiently to construct a multi-level PWM RF signal.
In many fields, for example in third and higher generation base stations, bandwidth-optimized modulation schemes are used for transmitting information. Bandwidth-optimized modulation schemes require a non-constant envelope, and thus have a relatively high peak-to-average power ratio (PAR). Linear power amplifiers such as class AB amplifiers are typically used because they offer high linearity. However, class AB amplifiers must be driven with a high back-off from the maximum (saturated) output power to achieve good linearity across a wide operating range. RF power amplifiers, of what ever class, e.g. type AB, or so-called switched-mode amplifiers of type D, E, F, etc., only achieve high efficiency when operated close to their maximum saturated power level. Thus, backing-off a class AB amplifier results in lower transmitted power, and thus reduced overall power efficiency.
Other conventional signal modulation techniques exist for Radio Frequency (RF) applications. However, each of the techniques suffers from poor power efficiency, poor linearity, complexity or other limitations, especially when the signal bandwidth is large. For example, supply voltage regulation techniques have poor power efficiency if the voltage regulator must have a large bandwidth. Linearity is problematic for Doherty amplifiers. Out-phasing, where two equally sized power amplifier outputs with an appropriately designed phase relation are combined via a power combiner, suffers from constant power dissipation. Delta sigma modulators used in conjunction with a high-power output stage tend to be less efficient than their PWM counterparts.
A pulse width modulator has been used to drive an RF power amplifier to impart amplitude signal modulations based on the duty cycle of the PWM signal applied to the RF amplifier. A conventional class AB or switched-mode RF power amplifier is driven into saturation by an input RF PWM signal, consisting of on-off bursts of an RF carrier with constant amplitude. The average burst duration (duty cycle) is made proportional to the baseband signal amplitude by so-called PWM coding. A way of generating a PWM signal is to compare, with a comparator circuit, the baseband signal to a threshold signal in the form of a triangle waveform with frequency fs, typically on the order of 10 times higher than the bandwidth B of the baseband signal. The comparator output signal is a baseband PWM signal, which is multiplied with an RF carrier, to form the RF PWM signal. In the case the RF carrier is phase modulation, the baseband signal fed to the comparator is typically the envelope of a complex baseband signal. When the baseband signal is below the threshold signal, there is ideally no input signal to, or output signal from, the RF power amplifier. The amplified RF PWM signal is then passed through a band-pass filter with bandwidth Bf, where typically B<Bf<fs, to remove the out-of-band power associated with the PWM, retaining only the desired amplified modulation RF signal. The efficiency of the RF power amplifier is ideally independent of the burst duration, apart from losses related to turning the amplifier on and off, which relatively become more significant the shorter the burst.
However, at lower baseband power levels, the PWM burst duration decreases, increasing the fraction of out-of-band versus in-band spectral power in the RF PWM signal. If the band-pass filter is connected directly to the amplifier output, it will typically present impedance terminations for the out-of-band spectral components which will prevent the amplifier from operating efficiently, if special care is not taken in the design of the amplifier, possibly leading to a more complex implementation. A circulator can be inserted between the amplifier output and the band-pass filter input, causing the out-of-band power to be reflected to the circulator dump port and dissipated into a matched load, without upsetting the amplifier. This has the drawback of reduced overall transmitter efficiency for low baseband signal levels (corresponding to short bursts), where the out-of-band power is high, although the RF amplifier itself is still operating efficiently.
SUMMARY OF THE INVENTIONAccording to the methods and apparatus taught herein, a baseband signal is processed by amplifying a first pulse width modulation radio frequency signal having a first non-negligible peak-to-peak amplitude and a second non-negligible peak-to-peak amplitude larger than the first non-negligible peak-to-peak amplitude. A second pulse width modulation radio frequency signal is also amplified, the second pulse width modulation radio frequency signal having a third non-negligible peak-to-peak amplitude approximately equal to the second non-negligible peak-to-peak amplitude of the first pulse width modulation signal when the baseband signal power is at or above the second threshold level. The amplified signals are constructively combined to form a pulse width modulation radio frequency signal comprising a plurality of non-negligible peak-to-peak amplitude levels each corresponding to a different baseband signal power range.
Of course, the present invention is not limited to the above features and advantages. Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.
The amplifier outputs are constructively combined to form an amplified PWM signal (VPWM
In more detail, the PWM input signal (VPWM
The combined output (VPWM
The amplifier outputs are coupled together by a transmission line coupler 320 including a first transmission line 322 and a second transmission line 324. The auxiliary amplifier 310 has a transmission line input 330 that compensates for phase shifting that occurs between the amplifier outputs. In another embodiment, the different transmission lines 322, 324, 330 can be replaced by an equivalent impedance-inverting network using inductors and capacitors, or transmission lines and capacitors, in a T or Pi network configuration. Regardless, the main amplifier 300 is driven by the main PWM input signal (VPWM
The auxiliary amplifier 310 is similarly driven by the first auxiliary PWM input signal (VPWM
The main amplifier 300 amplifies the main PWM input signal (VPWM
The auxiliary amplifier 310 is driven when the baseband signal voltage rises above ½ of the maximum baseband voltage. Both amplifiers 300, 310 are driven with relatively equal-amplitude PWM input signals when this condition occurs as shown in
Either way, the first comparator 500 compares the baseband signal (VBB) to a main modulation signal (VMOD
Regardless, the signal combiner 540 combines the first and second comparator outputs. The signal combiner output (VCOMP
The signal combiner output is input to the first multiplier 520 while the second comparator output (VCOMP2) is correspondingly input to the second multiplier 530. An RF carrier signal (VRF
In one embodiment, the first threshold corresponds to a baseband signal voltage of less than approximately 25% of the maximum voltage, i.e., when baseband power is less than approximately 1/16 of the total maximum power. The second threshold corresponds to a baseband signal voltage between approximately 25% and 50%, i.e., when baseband power is between approximately 1/16 and ¼ of the total maximum power. This way, the main amplifier 600 approaches continuous operation as the baseband signal power nears 1/16 of the total maximum power, reducing out-of-band spectral energy at even lower baseband power levels. The first auxiliary amplifier 610 begins to assist the main amplifier 600 when the baseband signal power exceeds 1/16 of the total power. The first auxiliary amplifier 610 approaches continuous operation as the baseband signal power approaches ¼ of the total power. The PWM circuitry 110 then activates the second auxiliary amplifier 620. The second auxiliary amplifier 620 approaches continuous operation as the baseband signal power nears the total maximum power.
The amplifier outputs are coupled together by a transmission line coupler 630 including first and second transmission lines 632, 634. The respective impedances of the transmission lines 632, 634 are selected so that the amplifier outputs can be constructively combined, and to ensure all amplifiers see load impedances enabling the amplifiers to operate in or near saturation (with maximum efficiency) at all three (e.g., 1/16, ¼, and 1× maximum) output power levels. The first and second auxiliary amplifiers 610, 620 each have a transmission line input 640, 650, respectively, that compensates for phase shifting that occurs between the amplifier outputs. First and second compensation transmission lines 660, 670 can be added to the outputs of the first and second auxiliary amplifiers 610, 620, respectively. The compensation transmission lines 660, 670 are selected to minimize the loading of the main amplifier 600 from the auxiliary amplifiers 610, 620 when they are not being driven. In another embodiment, the different transmission lines 632, 634, 640, 650, 660, 670 can be replaced by an equivalent impedance-inverting network using inductors and capacitors, or transmission lines and capacitors, in a T or Pi network configuration. Moreover, optional impedance transformers 680, 682, 684 can be included. A first impedance transformer 680 transforms the output impedance of the first auxiliary amplifier 610 while a second impedance transformer 682 transforms the output impedance of the second auxiliary amplifier 620. A third impedance transformer 684 transforms the total output impedance of the RF amplifier circuitry 120.
According to an embodiment, the first signal combiner 760 combines the first, second and third comparator outputs. The second signal combiner 770 similarly combines the second and third comparator outputs. The first combiner output (VCOMP
The second signal combiner output (VCOMP
The first signal combiner output (VCOMP
With the above range of variations and applications in mind, it should be understood that the present invention is not limited by the foregoing description, nor is it limited by the accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.
Claims
1. A method of processing a baseband signal, comprising:
- amplifying a first pulse width modulation radio frequency signal having a negligible peak-to-peak amplitude when the baseband signal power is below a first threshold level, a first non-negligible peak-to-peak amplitude when the baseband signal power is between the first threshold level and a second threshold level and a second, larger non-negligible peak-to-peak amplitude when the baseband signal power is at or above the second threshold level;
- amplifying a second pulse width modulation radio frequency signal having a negligible peak-to-peak amplitude when the baseband signal power is below the second threshold level and a non-negligible peak-to-peak amplitude approximately equal to the second non-negligible peak-to-peak amplitude of the first pulse width modulation signal when the baseband signal power is at or above the second threshold level; and
- constructively combining the amplified signals to form a third pulse width modulation radio frequency signal having a plurality of non-negligible peak-to-peak amplitude levels each corresponding to a different baseband signal power level.
2. The method of claim 1, wherein the first and second pulse width modulation radio frequency signals are generated by:
- comparing the baseband signal to a first modulation signal having a first peak-to-peak voltage range to obtain a first pulse width modulation signal;
- comparing the baseband signal to a second modulation signal having a second peak-to-peak voltage range to obtain a comparison signal and combining the comparison signal with the first pulse width modulation signal to obtain a second pulse width modulation signal;
- multiplying the first pulse width modulation signal with a first carrier signal to obtain the first pulse width modulation radio frequency signal; and
- multiplying the second pulse width modulation signal with a second carrier signal to obtain the second pulse width modulation radio frequency signal.
3. The method of claim 1, wherein constructively combining the amplified signals comprises coupling a transmission line between the amplified signals, the transmission line having an approximately ninety-degree phase shift at the carrier signal frequency.
4. The method of claim 1, further comprising filtering the third pulse width modulation radio frequency signal to obtain a radio frequency signal having at least two tones.
5. The method of claim 1, further comprising dissipating out-of-band frequency components of the third pulse width modulation radio frequency signal when the baseband signal power is relatively low.
6. A device, comprising:
- a first amplifier operable to amplify a first pulse width modulation radio frequency signal having a negligible peak-to-peak amplitude when the power of a baseband signal is below a first threshold level, a first non-negligible peak-to-peak amplitude when the baseband signal power is between the first threshold level and a second threshold level and a second, larger non-negligible peak-to-peak amplitude when the baseband signal power is at or above the second threshold level;
- a second amplifier operable to amplify a second pulse width modulation radio frequency signal having a negligible peak-to-peak amplitude when the baseband signal power is below the second threshold level and a non-negligible peak-to-peak amplitude approximately equal to the second non-negligible peak-to-peak amplitude of the first pulse width modulation radio frequency signal when the baseband signal power is at or above the second threshold level; and
- wherein the amplifier outputs are constructively combined to form a third pulse width modulation radio frequency signal having a plurality of non-negligible peak-to-peak amplitude levels corresponding to different baseband signal power ranges.
7. The device of claim 6, further comprising:
- a first comparator operable to compare the baseband signal to a first modulation signal having a first peak-to-peak voltage range to obtain a first pulse width modulation signal;
- a second comparator operable to compare the baseband signal to a second modulation signal having a second peak-to-peak voltage range to obtain a comparison signal;
- a combiner operable to combine the comparison signal with the first pulse width modulation signal to obtain a second pulse width modulation signal;
- a first multiplier operable to multiply the first pulse width modulation signal with a first carrier signal to obtain the first pulse width modulation radio frequency signal; and
- a second multiplier operable to multiply the second pulse width modulation signal with a second carrier signal to obtain the second pulse width modulation radio frequency signal.
8. The device of claim 7, wherein the radio frequency transmitter comprises a transmission line coupled between the amplifier outputs, the transmission line having an approximately ninety-degree phase shift at the carrier signal frequency.
9. The device of claim 6, further comprising a filter operable to filter the third pulse width modulation radio frequency signal to obtain a radio frequency signal having at least two tones.
10. The device of claim 6, further comprising a signal isolator operable to dissipate out-of-band frequency components of the third pulse width modulation radio frequency signal when the baseband signal power is relatively low.
11. A method of processing a baseband signal, comprising:
- generating a pulse width modulation radio frequency signal having a plurality of different non-negligible peak-to-peak amplitudes corresponding to different power ranges of the baseband signal; and
- filtering the pulse width modulation radio frequency signal to obtain a radio frequency signal having at least two tones.
12. The method of claim 11, wherein generating the pulse width modulation radio frequency signal comprises:
- driving first and second power amplifiers with pulse width modulation input signals of relatively equal non-negligible amplitude when the baseband signal power is above a first threshold level;
- driving only the first power amplifier with a pulse width modulation input signal having a reduced non-negligible amplitude when the baseband signal power is below the first threshold level; and
- constructively combining the amplifier outputs to form the pulse width modulation radio frequency signal.
13. The method of claim 12, further comprising:
- driving the first, second and a third power amplifier with pulse width modulation input signals of relatively equal increased non-negligible amplitude when the baseband signal power is at or above a second threshold level higher than the first threshold level; and
- driving only the first and second power amplifiers when the baseband signal power is between the first and second threshold levels.
14. The method of claim 12, wherein constructively combining the amplifier outputs comprises coupling a transmission line between the amplifier outputs, the transmission line having an approximately ninety-degree phase shift at a carrier frequency used to generate the pulse width modulation input signals.
15. The method of claim 11, wherein the pulse width modulation radio frequency signal is generated by a plurality of amplifiers whose outputs are coupled together.
16. The method of claim 11, further comprising dissipating out-of-band frequency components of the pulse width modulation radio frequency signal when the baseband signal power is relatively low.
17. A radio frequency transmitter, comprising:
- signal amplification circuitry operable to generate a pulse width modulation radio frequency signal having a plurality of different non-negligible peak-to-peak amplitudes corresponding to different power ranges of a baseband signal; and
- a filter operable to filter the pulse width modulation radio frequency signal to obtain a radio frequency signal having at least two tones.
18. The radio frequency transmitter of claim 17, wherein the signal amplification circuitry comprises:
- first and second power amplifiers operable to be driven with pulse width modulation input signals of relatively equal non-negligible amplitude when the baseband signal power is above a first threshold level, wherein only the first power amplifier is operable to be driven with a reduced non-negligible amplitude pulse width modulation input signal when the baseband signal power is below the first threshold level; and
- wherein the amplifier outputs are constructively combined to form the pulse width modulation radio frequency signal.
19. The radio frequency transmitter of claim 18, wherein the signal amplification circuitry further comprises a third power amplifier, wherein the first, second and third power amplifiers are operable to be driven with pulse width modulation input signals of relatively equal increased non-negligible amplitude when the baseband signal power is at or above a second threshold level higher than the first threshold level, and wherein only the first and second power amplifiers are operable to be driven when the baseband signal power is between the first and second threshold levels.
20. The radio frequency transmitter of claim 18, wherein the radio frequency transmitter comprises at least one transmission line coupled between the amplifier outputs, the transmission line having an approximately ninety-degree phase shift at a carrier frequency used to generate the pulse width modulation input signals.
21. The radio frequency transmitter of claim 17, further comprising a signal isolator operable to dissipate out-of-band frequency components of the pulse width modulation radio frequency signal when the baseband signal power is relatively low.
22. A radio frequency transmitter, comprising:
- first circuitry operable to sequentially drive different ones of a plurality of amplifiers with different ones of radio frequency pulse width modulation input signals as a function of baseband signal power;
- second circuitry operable to constructively combine the amplifier outputs to form a pulse width modulation radio frequency signal having a plurality of non-negligible peak-to-peak amplitude levels corresponding to different baseband signal power ranges; and
- a filter operable to filter the pulse width modulation radio frequency signal to obtain a radio frequency signal having at least two tones.
Type: Application
Filed: Sep 2, 2008
Publication Date: Mar 4, 2010
Patent Grant number: 8335250
Applicant: Infineon Technologies AG (Neubiberg)
Inventor: Johan Sjostrom (Tyreso)
Application Number: 12/202,768
International Classification: H03K 9/08 (20060101);